1. Field of the Invention
The present invention relates to a method for manufacturing a polycrysal semiconductor film used in a liquid crystal display device and the like, and more particularly to a method for manufacturing a polycrystal semiconductor film having crystal grains with a large and even diameter.
2. Description of the Related Art
Thin film transistors (hereinafter referred to as TFT) are normally formed on a polycrystal semiconductor film formed on a substrate such as a quarz substrate, a glass substrate or the like.
Semiconductor characteristics such as mobility of the polycrystal semiconductor film and the like are improved with an increase in the size of crystal grains. Consequently, in the case that a quarz glass having an excellent heat-resistance property is used as a substrate, and in the case that the damage of the substrate does not hinder the usage thereof, for example, such as a solar battery, there is used a method in which the substrate of an amorphous Si-film (hereinafter referred to as a-Si film) is heated as it is so that the semiconductor film is molten followed by holding the film in a heated state for a long time to anneal and to carry out a polycrystallization.
However, when the film is used as a TFT device of the liquid crystal display device, the quarz is very expensive with the result that the cost of the TFT becomes high. Consequently, the TFT device is consequently formed on the cheap glass substrate.
Here, when a polycrystal Si is used for the TFT device of the liquid crystal display device, a long-time high temperature annealing is required (for example heating for 8 to 56 hours in the atmosphere of a high temperature nitrogen at 600.degree. C. or more). However, in the case of the glass substrate, a deformation or a warp is generated.
Consequently, there is normally used a pulse laser irradiation method in which only the semiconductor film is heated and molten in a short time by applying an excimer pulse laser to the amorphous semiconductor film or the semiconductor film comprising a fine crystal polycrystal on the glass substrate with the result that a polycrystal film with a large grain size.
In other words, in the polycrystallization using an excimer laser, more than several 10 nanosecond pulses are irradiated to a surface of the semiconductor film such as the a-Si (amorphous Si) film which is deposited on the surface of the substrate to melt only the film and provide a solid state via a mixed state of a solid phase and a liquid phase thereby forming a polycrystal film.
However, as described above, the excimer laser annealing can melt only the semiconductor film such as the a-Si film or the polycrystal Si film by applying more than several 10 nano second laser pulse to the film surface. However, there arises a problem in that the heat dissipation to the substrate is very fast so that time up to the completion of solidification is short and the crystal grain cannot be grown to a large size. It is necessary to take as long time as possible up to the solidification to grow the crystal grains to an even and large diameter.
However, it is possible to prolong the time required for the solidification by prolonging the pulse length of the laser, and prolonging the heat input time with a multiple pulse using a plurality of lasers. For all that, a single pulse length can be prolonged only by several times, so that the time up to the completion of the solidification cannot be prolonged on a large scale. Furthermore, a multiple pulse method using a plurality of lasers is an effective method in principle, but controlling a plurality of unstable lasers at the same time was virtually impossible.
Furthermore, in a simple method, the solidification time can be prolonged to some extent by applying a laser in a state in which the substrate is heated to an extent which does not affect the glass (300 to 500.degree. C.). However, in the method, a large effect cannot be provided unless the substrate is heated to a temperature approximate to the melting point of the semiconductor. It was impossible to prolong the solidification time to a large extent up at a temperature of 300 to 500.degree. C. which is a heat resistant temperature in the usage of the glass substrate.
In this manner, the formation of the polycrystal semiconductor film using the conventional pulse laser has a problem in that the solidification time is extremely short until the end of the solidification and the crystal grains cannot be grown to a large size.
Another problem is that when the film is completely molten to the substrate or to the interface with the base film at the time of melting the semiconductor film such as a-Si film or the polycrystal Si film or the like with the excimer laser, crystal nuclei at the interface which constitute seeds of crystallization have disappeared and the molten liquid is supercooled at the time of cooling with the result that a large number of crystal nuclei are abruptly generated from the interface and from within the molten liquid and crystal grains with a large grain size are not generated, a polycrystal semiconductor film comprising a plurality of fine crystals is provided and semiconductor properties such as required mobility or the like are not obtained.
On the contrary when the melting of the semiconductor film is insufficient, a large number of crystal nuclei remain at an interface with the substrate or the base film so that a crystal growth proceeds with the residual crystal nuclei as seeds and fine crystal grains, which have a grain diameter d which is in an inverse proportion to the density (N) of the residual nuclei, are generated.
When a relation between the intensity of the laser beam and the residual nuclei is described, the density of the residual nuclei decreases with an increase in the intensity of the laser beam so that the size of the crystal increases until the intensity of the laser beam increases, the semiconductor film is completely molten and the residual nuclei completely disappear.
However, when the intensity of the laser beam attains a maximum limit, the residual nuclei disappear, and, the supercooling is generated in the process of cooling with the result that fine crystallization is generated.
Therefore, in order to generate large crystal grains in a uniform manner, the control of the density of the residual nuclei and the generation position of the nuclei are important. However, in the method for manufacturing the polycrystal semiconductor film by means of the conventional excimer laser annealing, a size of crystal grains which are generated largely change with a fine variation of the intensity of the laser beam in the vicinity of a maximum value so that the polycrystal semiconductor having a stable and uniform large crystal grains could not be supplied.